The present invention relates to the field of electron tubes and especially to travelling wave electron tubes working with millimeter waves (corresponding to frequencies of over 30 GHz).
Their working is based on an exchange of energy between a linear electron beam and a electromagnetic wave at a microwave frequency. The electron beam is emitted from a gun by a cathode. The cathode is placed at the entrance to a tubular interaction space. The electronic beam is long and thin, and travels through the interaction space. A focusing device surrounds the interaction space and confines the electrons of the beam to appropriate paths.
In the interaction space, the electron beam interacts with an electromagnetic wave at microwave frequency. The resulting amplified electromagnetic microwave is extracted by an appropriate device at output of the interaction space. The electron beam ends its journey in a collector placed at output of the interaction space.
The interaction space has a microwave circuit that is generally a helical delay line in the case of a travelling wave tube. It is taken to a potential that is generally a ground potential.
After having yielded a part of its energy to the electromagnetic microwave, the electron beam still possesses substantial kinetic energy when it penetrates the collector. The collector dissipates this energy in the form of heat.
Owing to difficulties of focusing the electron beam, these tubes work with low values of perveance. Their low interaction efficiency (in the range of 4% at 80 GHz) results in high beam energy values being needed and, hence high absolute values of cathode voltage that may exceed several tens of kilovolts. The low interaction efficiency also results in low variations in the speed of the electrons when they enter the collector.
To obtain a total efficiency greater than the interaction efficiency, a collector under low pressure is generally used. This collector comprises one or more electrodes taken to potentials greater than or equal to that of the cathode but below that of the interaction space. The closer this potential to the cathode, the greater will be the interaction efficiency.
In the case of a collector with several electrodes or a multistage collector, the electrodes are taken to potentials that decrease with distance from the interaction space, the last electrode being at a potential close to or equal to that of the cathode.
In these collectors, it is necessary to see to the electrical insulation of the different electrodes and to ensure that there is a discharge of the energy dissipated in the form of heat by the electrons that strike the electrodes.
Collectors of this kind may be of the clamped type. The electrodes inserted into a vacuum-tight envelope are held in position with respect to one another by means of longitudinal dielectric rods that provide the interface between the electrodes and the envelope.
These rods also electrically insulate the electrons from one another and from the envelope which is generally taken to the potential of the interaction space, namely the ground potential.
The thermal discharge is done radially from the electrodes to the envelope through the rods.
The increase in absolute value of the voltages of the electrodes leads to an increase in the diameter of the rods and to a decrease in their number. The consequent increasing of the radial space required by the collector is not desirable. The reducing of the contact surface between the rods and the electrodes on the one hand and between the rods and the envelope on the other hand has a deleterious effect on the removal of the heat.
The need to improve the quality of heat removal has led to the replacing of the rods with annular spacers placed between the electrodes, in contact on one side with the vacuum-tight envelope and on the other side with one or two electrodes.
The contact surface between the annular spacers and the electrodes and between the annular spacers and the envelope is increased and the circulation of the thermal flux takes place more efficiently from the electrodes towards the envelope.
The amount of space required remains unchanged. The higher the voltages of the electrodes in terms of absolute value, the larger the number of annular spacers.
The need to improve the cooling has led to the use a cooling device around the envelope. The device may be a device with fins and the cooling may be done by radiation. This structure is used especially in space applications. The cooling device may also be a fluid circulation device with the cooling being done by conduction.
The electrical connections of the electrodes are generally done longitudinally through the back of the collector because of the cooling device. The back of the collector is a hot zone because many electrons strike the electrode that is furthest from the interaction space. This is especially the case when the tube is not master controlled.
The solders needed at the electrical connections of the electrodes place limits on the maximum temperature acceptable for the back of the collector and hence on the power of the electron beam.
The present invention seeks to overcome these drawbacks.